Star tracker images are known to be corrupted by so-called single event upsets (SEU) appearing as points and streaks destroying the integrity of the image. The single event upsets are due to solar flares and other cosmic radiation and are particularly bothersome in missions approaching the sun.
Recently, imager chips of active pixel sensor (APS) type have been employed in star tracker apparatuses. Moreover, it has been proposed to integrate additional functionality with APS sensors in this application.
An example of such an image sensor specifically designed for star tracker applications is the so-called low cost low mass (LCMS) sensor developed by Cypress Semiconductor Corp. in collaboration with ESA. APS apparatuses of this type are provided with an on-chip logic circuit being capable of performing both a destructive readout of the optical pixels of the imager chip and a non-destructive readout of the pixels.
The core of the pixels is formed by photodiodes accumulating charge generated by irradiated photons. The destructive readout discharges the photodiodes of the pixels, whereas the non-destructive readout consists in measuring the transmissivity of a transistor whose gate voltage depends on the charge accumulated in the photodiode. As a consequence, the transistor acts as an amplifier for the pixel signal. Currently, the non-destructive read-out is used to perform a correlated double sampling which is used to minimize a fixed pattern noise (FPN) in the image signal.
Furthermore, the U.S. Pat. No. 7,145,188 B2 teaches to provide on-chip logic circuits on an imaging chip of CMOS type. It is proposed to perform a local adaptation of the pixel gain, if one or more pixels in a region are in saturation.
The object of the invention is to provide an active pixel sensor apparatus being capable of performing an on-the-fly processing of single event upsets and in particular, to enable an on-the-fly rejection of these single event upsets in order to reduce the complexity of the image processing in star tracker apparatuses.
The invention starts from an active pixel sensor apparatus for use in a star tracker device including an imager chip. The imager chip comprises an array of photodiodes operating as optical pixels. Moreover, the apparatus comprises a logic circuit being configured for reading out a pixel signal depending on an amount of light irradiated during a predetermined integration time and for resetting the optical pixel upon termination of the predetermined integration time. Moreover, the logic circuit is capable of processing the outputs of the optical pixels and to output the modified signals. Further, the logic circuit is configured for performing a non-destructive readout of the pixel signal during the integration time. In particular, the pixel may be reset and afterwards sampled and read out two or several times without additional reset in between.
One of the central points of the invention is to configure the logic circuit such that it may detect whether or not a discontinuity has occurred in the pixel signal during the integration time and to modify the signal depending on the result of this detection. The idea is based on the fact that the light of permanent light sources such as stars results in a continuous variation of the pixel signal, e.g. in a linear increase of the charge accumulated in the photodiode, whereas the single event upsets are characterized as a discontinuity in the form of a sudden jump of the signal during the integration time. The detection of the discontinuity therefore allows the identification of signals being affected by SEU and the discrimination of the different contributions to the signal, in particular the contribution of the continuous light sources from the contribution of the SEU. In order to remove the contribution of the SEU, it may be subtracted from the signal. In a further embodiment, the apparatus may be used as a lightening sensor when the modification of the signal consists in removing the continuous part instead of removing the discontinuous part.
A particularly small apparatus suitable for use in satellites can be achieved, if the logic circuit is an on-chip logic circuit provided on the imager chip.
Moreover, it is proposed that the logic circuit is further configured to periodically perform the non-destructive read-out during the integration time. This enables a quasi-continuous sampling of the signal in a discrete set of points. The discontinuity may be detected by comparing a difference between the results of subsequent non-destructive readouts with a predetermined threshold value. If the difference exceeds the threshold value, an SEU must have occurred between the last two non-destructive readout procedures.
In a preferred embodiment, the gain of the active pixels is set such that the brightest star in the field of vision of the apparatus generates a pixel signal just below the saturation value of the pixel. The threshold value should essentially correspond to the saturation value divided by the number of non-destructive readouts performed during one integration period.
In a particularly simple embodiment of the invention, the influence of the discontinuity may be removed, if the logic circuit is configured to determine an output value by repeatedly adding increments to a base value, wherein the base value may e.g. be zero. The logic circuit may then use the previously determined difference value between the results of subsequent non-destructive readouts in the same integration period as the increment, if this difference falls short of the predetermined threshold value, and use an extrapolated difference value as the increment, if the previously determined difference exceeds the predetermined threshold value.
If the logic circuit comprises storage means for storing at least one difference value, the logic circuit may determine the extrapolated difference value based on the stored difference value, e.g. by just copying the stored difference value or by performing some averaging procedure using previously determined difference values. If the extrapolated difference value is determined by averaging a certain number of stored difference values, the logic circuit may output a default value, if the actually stored number of previously determined difference values falls short of a predetermined minimum number of difference values being considered sufficient to perform the averaging.
The functionality of the logic circuit may be supplemented by a windowed read-out functionality of the image object, wherein only pixels within certain windows covering presumed positions of stars are read out in order to decrease the complexity of signal processing. Moreover, the logic circuit may perform correlated double sampling upon readout in order to avoid or reduce fixed pattern noise. Further, the logic circuit may subtract a background intensity level from the pixel signal.
In each of the above described embodiments, the active signal processing functionalities may be optional in the sense that they can be switched on and off. In order to allow for such a choice, the active pixel sensor apparatus may comprise means for activating or deactivating at least one of these signal processing functions of the logic circuit. The deactivation of unused function may result in a reduced energy consumption.
Further important features of the invention and the advantages thereof will be explained in the following description of the specific embodiment of the invention, which should be read in connection with the attached figures. The invention is not limited to the embodiment described below. The skilled person will easily find other combinations and sub-combinations of the characterizing features of the invention as defined in the appended claims.